The present invention relates to a press assembly and method of operating it to deform a workpiece.
There are many known press assemblies which are used to deform workpieces. these known press assemblies may have an upper die which cooperates with a lower die to deform the workpiece. Workpiece holders may be associated with the upper and/or lower dies. Press assemblies having known constructions are disclosed in U.S. Pat. Nos. 1,884,700; 2,217,172; 3,296,850; 3,456,478; 3,636,748; and 3,636,749.
Stretch drawing is performed during the operation of some known presses. During stretch drawing, the edges of a sheet metal workpiece are firmly gripped between upper and lower draw rings. After the workpiece has been gripped, an upper die is closed against a lower die to deform the workpiece. Under certain circumstances, a stretch draw operation is preferred because it reduces the total amount of metal used to form an article, tends to improve the quality of the article, and facilitates maintaining uniform quality during production of a series of articles.
Although stretch draw operations are preferred during the forming of certain articles, there are problems which may be encountered during a stretch draw operation. These problems relate to an extremely high noise level and to damage which occurs due to shock loading. Shock loading has been so bad that, in the past, a crank shaft on a large press assembly has broken under the influence of the shock loading. In addition, there is a tendency for cushion assemblies and other components of the press assembly to loosen due to the severe vibrations to which the press assembly is subjected. It is believed that the severe shock loading and vibrations during a stretch draw operation will substantially decrease the total operating life of a press assembly.
When a press assembly is operated from an open condition to a closed condition during a stretch draw operation, an upper draw ring and workpiece are impacted against a lower draw ring. When this occurs, extremely large shock loading forces may be present. Thus, the lower draw ring may have a weight of between 1,400 and 70,000 pounds. When an upper draw ring impacts against a stationary lower draw ring at a speed of approximately 100 feet per minute, the inertia of the lower draw ring resists acceleration. Acceleration of the lower draw ring is also resisted by upwardly directed biasing forces applied against the lower draw ring by cushion assemblies.
When the press assembly is operated from a closed condition to an open condition, the upper and lower draw ring move upwardly together. When the lower draw rings reaches the end of its upward stroke, it is moving upwardly at a substantial speed, for example, at a speed of approximately 100 feet per minute. Due to the inertia of the lower draw ring, it may separate from its cushion assemblies by a distance of approximately one inch. The lower draw ring then falls back on the cushion assemblies which then cause the draw ring to rebound. Downward falling and rebounding of the draw ring is repeated, with decreasing strokes, as the press continues to open. The rebounding of the lower draw ring on the cushion assemblies vibrates the cushion assemblies in such a manner that they tend to become loose and leak. The vibrating load, to which the lower draw ring subjects the cushion assemblies, can result in substantial wear of components of the cushion assemblies.
In an effort to avoid shock loading of components of a press assembly during operation of the press assembly from an open condition to a closed condition, U.S. Pat. No. 4,499,750 suggests that a lower draw ring or sheet metal holder be accelerated in a downward direction before an upper draw ring or drawing die comes into contact with the workpiece. Downward acceleration of the lower draw ring is effected by actuating a control valve in timed relationship with opening of the press. Actuation of the control valve exposes a piston to fluid pressure to move the lower draw ring downwardly against the influence of a die cushion piston.